The long term objective of this proposal is to understand the events in B cell development and selection that lead to the production of autoantibodies in MRL/Mp-lpr/lpr (MRL/lpr) mice. Mice of this strain develop a spontaneous autoimmune disease that resembles systemic lupus erythematosus (SLE). We have begun a study of the B cell response to the Sm particle, a ribonucleoprotein present in the nuclei of all cells. The spontaneous response to this particle in humans is diagnostic of SLE, and MRL/lpr mice are the only mouse model that spontaneously develops a response to this antigen. The correlation of the response to Sm and SLE suggests an essential relationship between the etiology of the disease and the production of these autoantibodies. Our previous analysis indicates that Sm-specific B cells are selected by DNA, but also indicates the involvement of a second antigen, presumably Sm. We propose in Aim 1 to test the hypothesis that Sm is a selecting antigen in this response. This will be accomplished by identifying the mutations in multiple anti-Sm hybridomas, and determining whether their distribution is biased, an indication of antigen selection of mutant B cells. In addition, through the use of transfectoma antibodies we will determine whether the observed mutations improve Sm and DNA binding. In Aim 2 we will examine the basis for the dual Sm and DNA binding of anti-Sm selected hybridomas. We propose that DNA binding is determined principally by the H chain and that Sm binding is determined principally by the L chain. This hypothesis will be tested by measuring Sm and DNA binding of generated transfectomas antibodies that differ in the VH or Vk. In Aim 3 we will generate transgenic mice using VH and Vk genes of anti-Sm and anti-Sm/DNA hybridomas. Transgenic mice will be crossed onto both normal and autoimmune genetic backgrounds to examine the immunoregulation of these cells in normal mice and their disregulation in autoimmune mice.